1. Field of the Invention
The present invention relates to a multimedia broadcast/multicast service (MBMS) system. More particularly, the present invention relates to a method and apparatus for dispersing user equipments (UEs) to non-preferred frequencies using time information for session start notification in a MBMS system.
2. Description of the Related Art
With developments of various communication technologies, mobile communication systems using a wideband code division multiple access (CDMA) scheme are developing into systems for multimedia broadcast/communication capable of providing a conventional voice service, as well as a multimedia service and a packet communication service for transmitting large amounts of data. To support the multimedia broadcast/communication, MBMS service provided from one or more multimedia data sources to a plurality of user equipments (UEs) is being implemented.
The term “MBMS service” refers to a service for transmitting through a wireless network the same multimedia data to a plurality of receivers. As a result, radio transmission resources can be efficiently used because the receivers share one radio channel. The MBMS service supports the transmission of multimedia, such as, realtime video and voice, still images, text, and the like. The MBMS service, which requires a large amount of resources for transmission, simultaneously transmits voice data and video data according to a type of multimedia transmission. Because an MBMS service must transmit the same data to a plurality of cells in which users are located, a Point-to-Point (PtP) or Point-to-Multipoint (PtM) connection is made according to the number of users located in each cell.
Recent MBMS systems are supporting frequency layer convergence (FLC) technology for allocating a preferred frequency for each of the MBMS services provided, and allowing UEs to receive desired MBMS services in a preferred frequency layer (PL). FLC information indicates preferred frequencies mapped to MBMS services. MBMS FLC is a method for moving UEs to a preferred frequency for each service such that one service can be provided at only one frequency, rather than all frequencies overlapped in an identical area.
FIG. 1 illustrates a conventional structure of cell layers overlapped in an identical service area and an example of MBMS FLC.
FIG. 1 illustrates a cell structure in which different frequencies, that is, Frequency 1, Frequency 2, Frequency 3, and Frequency 4, are overlapped in an identical area. Reference numerals 101, 111, 121, and 131 comprise Frequency 1, Frequency 2, Frequency 3, and Frequency 4, respectively. Reference numeral 141 comprises a controlling radio network controller (CRNC) for controlling cells of the frequencies. FLC is a method for moving UEs to a preferred frequency for each service such that each service can be provided at only one frequency, without providing one service from all cells of Frequency 1, Frequency 2, Frequency 3, and Frequency 4 overlapped in an identical area, as illustrated in FIG. 1.
In FIG. 1, the CRNC 141 receives a session start message, which notifies that a specific MBMS session will be started from a serving General Packet Radio Service (GPRS) support node (SGSN) (not illustrated) over an Iu interface in Process 1. When receiving the session start message, the CRNC 141 selects a preferred frequency for an MBMS service in Process 2. In FIG. 1, an assumption is made that Frequency-2 111 has been selected as the preferred frequency for the service session. In Process 3, information of Preferred Frequency-2 111, for the MBMS service selected in Process 2, is sent to cells of all frequencies, that is, Frequency-1 101, Frequency-2 111, Frequency-3 121, and Frequency-4 131.
Among the UEs requesting to receive the MBMS service session after receiving the preferred frequency information, UEs at non-preferred frequencies 101, 121, and 131 move to the preferred frequency 111 in Process 4. The UEs at the preferred frequency 111 continuously remain at the preferred frequency 111. The above-described Process 4 is referred to as the FLC. The UEs requesting to receive the service session, according to the FLC, concentrates at the preferred frequency 111.
When the session ends, as described with reference to FIG. 1, the UEs concentrated at the preferred frequency 111 need to be dispersed to many different frequencies.
Accordingly, there is a need for an improved method and apparatus that disperses the UEs, which are concentrated at a preferred frequency, to many different frequencies in a MBMS system.